Abstract
The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.
Highlights
Photocatalysis is currently at the center of scientific interest as a potential route to efficient light harvesting for the production of transportable fuels.[1−3] transfer of photocatalysis into applications is hampered by the low quantum efficiency of the whole process, which typically does not exceed 10%
The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations
Coverage-dependent investigations on the rutile (110) surface showed that the photoactivity of methanol decreases with an increasing coverage.[20] (This behavior is opposite to the results on anatase, presented in this work.) It was further reported that the formation of methoxy species significantly increases the photocatalytic activity.[8]
Summary
Photocatalysis is currently at the center of scientific interest as a potential route to efficient light harvesting for the production of transportable fuels.[1−3] transfer of photocatalysis into applications is hampered by the low quantum efficiency of the whole process, which typically does not exceed 10%. It is worth mentioning that the coverage dependence of methanol photocatalysis on the anatase (101) surface reported in this work is exactly the opposite of that found on the rutile (110) surface.[20] There it was reported that the methanol photoactivity drops above 0.67 ML; this effect was attributed to methanol molecules hydrogen-bonded to surface bridging O2c atoms.[20] Further, a computational study showed that the energy balance for methanol dissociation on rutile turns in favor of the undissociated configuration at higher coverages,[49] again opposite to the anatase case This strongly indicates that the hydrogenbonding network within the first monolayer plays a key role in photocatalysis, and the exact arrangement of the hydrogen bonds is determined by the surface structure
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